The studies on this project are currently focusing on developing a new method for tracking sperm during transport in the female reproductive tract. The preliminary studies have been conducted in rabbits, therefore no macaques were used on this project during the past year. The new methodology involves labeling ejaculated sperm with gadolinium. The labeled sperm are subsequently used for artificial insemination of a female and the sperm are monitored in the reproductive tract using MRI technology. Preliminary studies have demonstrated that rabbit sperm survive well for 16 h after labeling with gadolinium with only a modest reduction in percentage of motile sperm compared to unlabeled sperm. *KEY* Macaque, Sperm antigen, Immunocontraception